Pharmacokinetic analysis of the interaction between dicoumarol and tolbutamide in man

The effect of repeated administration of tolbutamide on the elimination and anticoagulant action of a single oral dose of dicoumarol 600 mg was studied in four healthy male subjects using a crossover design. In all subjects the plasma concentration of dicoumarol in the postabsorptive phase was lower during concomitant tolbutamide treatment. However, the subjects differed with respect to the elimination kinetics of dicoumarol and the effect of tolbutamide on some of the measured pharmacokinetic paramaters. In two subjects dicoumarol was eliminated by apparent first-order kinetics. Tolbutamide led to a pronounced increase in the elimination rate and a shift in the plasma concentration-response relationship towards a lower concentration of dicoumarol. The total hypoprothrombinaemic effect per dose of dicoumarol was not affected. The decline in the dicoumarol concentration in plasma in the other two subjects was concentration-dependent. Apparent first-order kinetics were observed only at plasma concentrations below 10 mg/L. Tolbutamide treatment did not markedly affect the slope of the terminal portion of the plasma concentration vs. time curve, but diminished the area under the total curve. The plasma concentration-response relationship of dicoumarol was not affected by tolbutamide, but there was a small decrease in the area under the anticoagulant effect vs. time curve. The plateau level of tolbutamide in plasma increased considerable in all subjects after administration of one dose of dicoumarol. Thus, simultaneous administration of tolbutamide and dicoumarol to man often causes no changes in the anticoagulant activity of dicoumarol, but this is due not to lack of interaction of the drugs but to the complexity of their interactions, involving processes that may counteract each other.